The state of the Oligocene icehouse world: Sedimentology, provenance, and stable isotopes of marine sediments from the Antarctic continental margin.

摘要

The Oligocene Epoch (34-23 Ma) was a dynamic time in Antarctica, with previous ice volume estimates suggesting fluctuations from below 50 % up to 140 % of modern all while atmospheric CO2 decreased from above 1,000 ppm in the Early Oligocene to near modern levels by the Late Oligocene. Most of what is known about the Oligocene Antarctic cryosphere however, is derived from distal sedimentary records that can only provide a generalized view of the cryospheric dynamics in Antarctica. To better understand regional differences in Antarctic glacial dynamics, proximal records are needed. This dissertation advances our understanding of these dynamics in Antarctica during the Oligocene by investigating three proximal, marine sediment cores from different regions of the continent.;Ice-rafted debris (IRD) concentrations, 40Ar/39 Ar thermochronology, and stable isotope records combined from 3 proximal marine sediment cores reveal a large ice sheet existed throughout the Oligocene, with ice volume reaching up to 155 % of modern. Concentrations and 40Ar/39Ar thermochronology from IRD offshore of the Wilkes subglacial basin suggest the ice sheet was fairly stable on elevated portions such as the Adelie Craton, but the basin itself was more responsive to climate changes. These changes appear to be influenced by 405-kyr eccentricity and 1.2 myr obliquity. In the Ross Sea, 40Ar/39Ar thermochronology from IRD show a large West Antarctic influence, indicative of a large ice sheet residing there during the Late Oligocene. Stable isotopes from benthic foraminifera from the Maud Rise show ice volume fluctuations from below 50 % up to 155 % of modern, in agreement with modeling and far-field records. The isotope record is also influenced by 405-kyr and 100-kry eccentricity and does not show a warming trend during the Late Oligocene as seen in other isotope records. Together, these records are indicative of a near-modern size or larger ice sheet present in both East and West Antarctica during the Oligocene, a time when the extent of Antarctica glaciation has been debated.